Thermodynamic parameters for the complexation of technetium(IV) with EDTA

Abstract

<jats:title>Abstract</jats:title> <jats:p>Technetium-99 is a high yield (~6% fission yield) fission product and long-lived (2.13×10<jats:sup>5</jats:sup> year half-life) component of nuclear waste that will be disposed of in a geological repository. Some <jats:sup>99</jats:sup>Tc has been released into the environment due to nuclear fuel and weapon production activities at sites such as Hanford, WA. Strongly complexing ligands such as ethylenediamine-<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>,<jats:italic>N</jats:italic>′,<jats:italic>N</jats:italic>′-tetraacetic acid (EDTA) are known to increase Tc(IV) solubility and mobility in environmental systems and an accurate quantification of the complexation of Tc(IV) with EDTA is important for predicting its behavior in a geological repository. A liquid–liquid extraction system utilizing 0.2 M TOPO in dodecane was used to measure the stability constants of Tc(IV)-EDTA in 0.50 <jats:italic>m</jats:italic> NaNO<jats:sub>3</jats:sub> at variable temperatures (14.0±0.1, 25.0±0.1, and 32.0±0.1°C). The acid dependence of the apparent stability constants in the p<jats:italic>C</jats:italic><jats:sub>H</jats:sub> range of 2.00–2.70 indicated the formation of TcO(EDTA)<jats:sup>2−</jats:sup> (log<jats:italic>β</jats:italic><jats:sub>101</jats:sub>=17.9±0.3, 25.0±0.1°C) and a protonated complex TcO(H)(EDTA)<jats:sup>−</jats:sup> (log<jats:italic>β</jats:italic><jats:sub>111</jats:sub>=20.5±0.1, 25.0±0.1°C). The associated thermodynamic parameters Δ<jats:sub>r</jats:sub><jats:italic>G</jats:italic><jats:sub>101</jats:sub>=−101.7±0.4 kJ·mol<jats:sup>−1</jats:sup>, Δ<jats:sub>r</jats:sub><jats:italic>H</jats:italic><jats:sub>101</jats:sub>=−47±9 kJ·mol<jats:sup>−1</jats:sup>, Δ<jats:sub>r</jats:sub><jats:italic>S</jats:italic><jats:sub>101</jats:sub>=179±36 J·mol<jats:sup>−1</jats:sup>·K<jats:sup>−1</jats:sup>, Δ<jats:sub>r</jats:sub><jats:italic>G</jats:italic><jats:sub>111</jats:sub>=−117.2±0.3 kJ·mol<jats:sup>−1</jats:sup>, Δ<jats:sub>r</jats:sub><jats:italic>H</jats:italic><jats:sub>111</jats:sub>=−23±5 kJ·mol<jats:sup>−1</jats:sup>, and Δ<jats:sub>r</jats:sub><jats:italic>S</jats:italic><jats:sub>111</jats:sub>=315±63 J·mol<jats:sup>−1</jats:sup>·K<jats:sup>−1</jats:sup> (0.50 <jats:italic>m</jats:italic> NaNO<jats:sub>3</jats:sub>, 25.0±0.1°C) were determined by van’t Hoff analysis. The formation of each Tc(IV)–EDTA complex is exothermic and present favorable entropy terms.</jats:p>

Technetium Complexation with Multidentate Carboxylate-Containing Ligands: Trends in Redox and Solubility Phenomena

“…A more recent study proposed the formation of two Tc–EDTA complexes: log K °(TcO­(EDTA) 2– ) = (20.0 ± 0.4) and log K °(Tc­(OH)­(EDTA) − ) = (25.3 ± 0.5). These values were later updated in another study (log K °(TcO­(EDTA) 2– ) = (17.9 ± 0.3) and log K °(Tc­(OH)­(EDTA) − ) = (20.5 ± 0.1)). Both publications reported that these complexes prevail in acidic to near-neutral pH conditions, but the reported stability constants were calculated using the TcO 2+ moiety recently excluded in the NEA-TDB.…”

“…While investigations of technetium interactions with most of these ligands have been reported within the literature, − a majority were conducted under acidic conditions and thus may not be representative of the type of complexes forming in alkaline systems. For instance, the strong hydrolysis of Tc­(IV) results in the predominance of TcO­(OH) 3 – at pH ≥ 10, which may outcompete the formation of binary Tc­(IV)–L complexes or contribute to the formation of ternary Tc­(IV)–O/OH–L complexes.…”